Micro-analyzer with passive aggregator

ABSTRACT

A micro-analyzer is described. This micro-analyzer includes an outer surface region on a sampler surface that receives liquid droplets, and aggregates and moves the droplets radially toward an inner surface region on the sampler surface that receives the droplets. For example, the outer surface region may include a set of micro-patterned concentric rings, each of which includes a set of radially oriented wall-groove pairs. Moreover, the sampler surface may be increasingly less hydrophobic along a radial direction toward the center of the sampler surface, thereby creating an axisymmetric wettability gradient. After the droplets are aggregated, an analysis mechanism in an area within the inner surface region performs analysis on the aggregated droplets.

BACKGROUND

1. Field

The present disclosure generally relates to a micro-analyzer. Morespecifically, the present disclosure relates to a micro-analyzer with apassive aggregator for aggregating liquid droplets.

2. Related Art

Exhaled human and animal breath analysis has become attractive as adiagnostic tool, for example, for various diseases including: cancer,asthma, and respiratory infections. One of the technological challengesfor collecting exhaled breath samples from subjects is the design of anefficient and reliable breath sampler. Existing breath-samplingtechniques for collecting exhaled breath are typically power hungry,bulky, and have wide variability in performance (such as reproducibilityand reliability). Moreover, the breath samples collected using theexisting breath-sampling techniques are usually manually processed,which increases the risk of contamination and handling errors.

Hence, what is needed is a micro-analyzer without the problems describedabove.

SUMMARY

One embodiment of the present disclosure provides a micro-analyzer. Thismicro-analyzer includes: a substrate; and a sampler surface disposed onthe substrate that receives liquid droplets. Moreover, the samplersurface includes: an outer surface region that facilitates aggregatingand moving the droplets radially toward a center of the sampler surface;and an inner surface region, within the outer surface region, whichreceives the droplets collected by the outer surface region.Furthermore, the inner surface region includes an area with an analysismechanism configured to analyze the droplets received at the innersurface region.

Note that the sampler surface may be increasingly less hydrophobic alonga radial direction toward the center of the sampler surface. Forexample, the outer surface region may be micro-patterned to create awettability gradient which is distributed axisymmetrically.(Alternatively, the wettability gradient may be distributednon-symmetrically.) Moreover, the wettability gradient may increase inan inward radial direction.

In some embodiments, the outer surface region is micro-patterned tocreate a wettability gradient which is distributed as a space-filledgeometric hierarchically repeating pattern or which is distributed as aspace-filled fractal pattern.

Furthermore, the outer surface region may include a set ofmicro-patterned concentric rings. A given micro-patterned concentricring may include a set of radially oriented wall-groove pairs. Forexample, a wall within a given wall-groove pair may have a trapezoidalcross-section when viewed from above, and/or a base of the wall may havea varying width which increases in the inward radial direction.

In some embodiments, the number of wall-groove pairs in the givenmicro-patterned concentric ring increases from the outermost ring towardthe innermost ring. For example, the number of wall-groove pairs mayincrease as a result of a decreasing width of the grooves from theoutermost ring toward the innermost ring. Alternatively, the number ofwall-groove pairs may increase as a result of a decreasing width of thegrooves from one side of a non-symmetrically configured device toanother. Note that the number of micro-patterned rings may control awettability gradient of the outer surface region.

In some embodiments, the sampler surface receives a gas-phase orvapor-phase sample that condenses into the liquid droplets. Inparticular, the outer surface region may facilitate condensing at leasta component in the received gas-phase sample into liquid-phase dropletson the sampler surface.

Additionally, the micro-analyzer may include a hydrophobic coatingdisposed on the outer surface region (and/or the inner surface region)to facilitate drop-wise condensation. This hydrophobic coating mayinclude a super-hydrophobic thin film.

In some embodiments, the micro-analyzer includes an active condensationmechanism that continuously cools the sampler surface to facilitatedrop-wise condensation. Alternatively or additionally, the activecondensation mechanism may adaptively cool the sampler surface tofacilitate selected condensation of pre-defined biomarkers withincertain temperature regimes. For example, the active condensationmechanism may include a thermoelectric cooler.

Note that the substrate may be circular or non-circular. Moreover, thesubstrate may include silicon or a material other than silicon.

In some embodiments, the gas-phase sample includes a breath sample thatincludes at least a gaseous-phase component and/or an aerosolizeddroplet (which may include non-volatile compounds).

Another embodiment provides a method for analyzing droplets using themicro-analyzer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram illustrating a side view of a micro-analyzerin accordance with an embodiment of the present disclosure.

FIG. 2 is a block diagram illustrating a top view of the micro-analyzerof FIG. 1 in accordance with an embodiment of the present disclosure.

FIG. 3 is a flow diagram illustrating fabrication of the micro-analyzerof FIGS. 1 and 2 in accordance with an embodiment of the presentdisclosure.

FIG. 4 is a flow chart illustrating a method for analyzing droplets inaccordance with an embodiment of the present disclosure.

Table 1 provides the geometry and static contact angles for sets ofmicro-patterned concentric rings in the micro-analyzer of FIGS. 1 and 2in an embodiment of the present disclosure.

Note that like reference numerals refer to corresponding partsthroughout the drawings. Moreover, multiple instances of the same partare designated by a common prefix separated from an instance number by adash.

DETAILED DESCRIPTION

Embodiments of a micro-analyzer and a method for analyzing dropletsusing the micro-analyzer are described. This micro-analyzer includes anouter surface region on a sampler surface that receives liquid droplets,and aggregates and moves the droplets radially toward an inner surfaceregion on the sampler surface that receives the droplets. For example,the outer surface region may include a set of micro-patterned concentricrings, each of which includes a set of radially oriented wall-groovepairs. Moreover, the sampler surface may be increasingly lesshydrophobic along a radial direction toward the center of the samplersurface, thereby creating an axisymmetric wettability gradient. Afterthe droplets are aggregated, an analysis mechanism in an area within theinner surface region performs analysis on the aggregated droplets.

The micro-analyzer may facilitate portable, energy-efficient gas-phasesamplers, such as exhaled breath samplers, with high reproducibility andreliability. For example, the outer surface region may facilitatecondensing at least a component in a received gas-phase sample intoliquid-phase droplets on the sampler surface, which are then collectedand analyzed by the analysis mechanism. Moreover, the micro-analyzer maybe fabricated with high yield and low cost. Furthermore, the integratedmicro-analyzer may eliminate the need for manual processing of thedroplets, which may reduce or eliminate the risk of contamination andhandling errors.

We now describe embodiments of a micro-analyzer. FIG. 1 presents a blockdiagram illustrating a side view of a micro-analyzer 100. Thismicro-analyzer includes: a substrate 110;

and a sampler surface 112 (such as a patterned surface) disposed onsubstrate 110 that receives liquid droplets 116 (FIG. 2). For example,droplets 116 (FIG. 2) may be provided using a dropper or a pipette.

Moreover, as shown in FIG. 2, which presents a top view ofmicro-analyzer 100, sampler surface 112 includes: an outer surfaceregion 114 that facilitates aggregating and moving droplets 116 radiallytoward a center 118 of sampler surface 112; and an inner surface region120, within outer surface region 114, which receives droplets 116collected by outer surface region 114.

Note that sampler surface 112 may be increasingly less hydrophobic alonga radial direction 122 toward center 118 of sampler surface 112. Forexample, outer surface region 114 may be micro-patterned to create awettability gradient which is distributed axisymmetrically.(Alternatively, the wettability gradient may be distributednon-symmetrically.) Moreover, the wettability gradient may increase inan inward radial direction. In some embodiments, outer surface region114 is micro-patterned to create a wettability gradient which isdistributed as a space-filled geometric hierarchically repeating patternor which is distributed as a space-filled fractal pattern.

Furthermore, outer surface region 114 may include a set ofmicro-patterned concentric rings 124. A given micro-patterned concentricring (such as micro-patterned concentric ring 124-1) may include a setof radially oriented wall-groove pairs (such as wall-groove pair 126).For example, a wall 128 within a given wall-groove pair may have atrapezoidal cross-section when viewed from above, and/or a base of wall128 may have a varying width 130 which increases in the inward radialdirection. Thus, in some embodiments the width 130 increases towards thecenter of sampler surface 112. This increase in area may provide thewettability gradient.

In some embodiments, the number of wall-groove pairs in the givenmicro-patterned concentric ring increases from outermost ring 124-1toward innermost ring 124-N. For example, the number of wall-groovepairs may increase as a result of a decreasing width 130 of the groovesfrom outermost ring 124-1 toward innermost ring 124-N. Alternatively,the number of wall-groove pairs may increase as a result of a decreasingwidth 130 of the grooves from one side of a non-symmetrically configureddevice to another. Note that the number of micro-patterned rings maycontrol a wettability gradient of outer surface region 114.

Additionally, micro-analyzer 100 may include analysis mechanism 136 inan area 138 in inner surface region 120. This analysis mechanism mayanalyze the aggregated droplets. For example, micro-analyzer 100 mayperform on-chip micro-analysis. In particular, analysis mechanism 136may perform a wide variety of analysis techniques, including: chemicalanalysis techniques (such as gas chromatography, mass spectrometry,chemical detection, etc.), materials analysis techniques, proteinanalysis techniques and blood-cell analysis techniques (such as bloodcytology). While FIG. 2 illustrates an analysis mechanism integrated inmicro-analyzer 100, in other embodiments the analysis mechanism may beexternal to micro-analyzer 100. In this way, micro-analyzer 100 may beused in conjunction with a variety of analyzers, including thoseprovided by third parties.

In some embodiments, sampler surface 112 receives a gas-phase samplethat condenses into liquid droplets 116. In particular, outer surfaceregion 114 (and/or inner surface region 120) may facilitate condensingat least a component in the received gas-phase sample into liquid-phasedroplets on sampler surface 112. These droplets may then be passivelyaggregated and moved towards analysis mechanism 136.

Referring back to FIG. 1, micro-analyzer 100 may include a hydrophobiccoating 132 disposed on outer surface region 114 (FIG. 2) and/or innersurface region 120 (FIG. 2) to facilitate drop-wise condensation inembodiments where a gas-phase sample is received. This hydrophobiccoating may include a super-hydrophobic thin film (which may have acontact angle with water greater than 150°).

In some embodiments, micro-analyzer 100 includes an optional activecondensation mechanism 134 that continuously cools sampler surface 112to facilitate drop-wise condensation. Alternatively or additionally,optional active condensation mechanism 134 may adaptively cool samplersurface 112 to facilitate selected condensation of pre-definedbiomarkers within certain temperature regimes or ranges. For example,optional active condensation mechanism 134 may include a thermoelectriccooler.

Note that substrate 110 may be circular or non-circular. For example,while a rotationally symmetric geometry is illustrated in FIGS. 1 and 2,in other embodiments sampler surface 112 may include micro-patternedtrenches arranged in a rectangular geometry. Moreover, substrate 110 mayinclude silicon or a material other than silicon. Therefore, in someembodiments substrate 110 includes a material such as: glass, silicon, aceramic, and/or a plastic (for example, substrate 110 may be fabricatedusing injection-molded plastic). In some embodiments, micro-analyzer 100is a passive component.

Micro-analyzer 100 may be used in a wide variety of applications,including: power-generating systems, thermal management (such as coolingof electronics), two-phase flow devices, self-cleaning devices,environmental ambient sampling (such as detection of aerosols, pathogensand/or chemicals in the atmosphere), micro-fluidics, forensics,breath-sample collection for drug testing (including drunk driving, useof prescription medication, use of recreational or illicit, drugs, useof drugs or abuse such as cocaine, marijuana or methamphetamine, etc.),and biological-sample collection for use in medical testing. Forexample, the medical testing may be associated with a disease, such as:an infection, asthma, tuberculosis, chronic obstructive pulmonarydisease, chronic respiratory disease, cancer, liver disease, etc. Moregenerally, the medical testing may be used to monitor or assess themetabolism of an individual, including non-disease monitoring.

Other applications include assessing: the exposure of an individual toexogenous materials, chemicals or toxins (including those that areingested, absorbed through the skin or breathed in); an individual's oran animal's metabolism of a specific drug or chemical; stress biomarkersassociated with a psychological state (such as whether an individual islying or is mentally ill); physiological biomarkers associated withreproduction (such as hormones); compliance of an individual with amedication regime (such as, did the individual take their medicine atthe appropriate times); whether a prescribed medication is attherapeutic dose); intentionally ingested taggants or tracking compounds(which may be used to identify and/or authenticate an individual);biometric identification; and/or biomarkers related to sensitivity of anindividual to operating conditions (such as a susceptibility to poorperformance at high altitude or alertness). For example, this approachmay be used to assess dietary or travel habits of an individual based onthe chemicals detected in the breath. Alternatively, this approach maybe used to assess the short- and long-term effects of radiationexposure. In some embodiments, micro-analyzer 100 is used in a singlepoint-of-care device (such as a ‘lab on a chip’) that is used todiagnose or monitor the health of an individual, an animal or a plant.

In an exemplary embodiment, micro-analyzer 100 is used to condense,transport and aggregate droplets 116 (FIG. 2) from a breath samplereceived from an individual (i.e., the gas-phase sample includes abreath sample that includes at least a gaseous-phase component and/or anaerosolized droplet that may include non-volatile compounds). Inparticular, micro-analyzer 100 includes a microelectromechanical system(MEMS)-based exhaled breath sampler for the capture of both volatile andnon-volatile biomarker metabolites. The surface of the sampler (forexample, sampler surface 112) may promote drop-wise condensation, andmay enable a free-energy-driven mechanism to collect exhaled breathcondensate from the surface. For example, drop-wise condensation may beenhanced by making the surface of the sampler super-hydrophobic. Notethat drop-wise condensation may be preferable over film-wisecondensation because it allows one order of magnitude largercoefficients of heat transfer between the surface and the exhaledbreath.

Moreover, the surface of the sampler may be patterned with aradially-distributed wettability gradient that provides afree-energy-driven mechanism to route exhaled breath condensate dropletstoward a central collection point. Therefore, the transport of thedroplets on the surface may be passive (i.e., a power source may not beused to transport the droplets on the surface). Furthermore, thewettability gradient may contribute to maintaining drop-wisecondensation on the sampler surface by continuously removing dropletsfrom the surface, and thus freeing prior nucleation sites for newdroplets to nucleate.

The wettability of a flat surface is described by Young's Equation

$\begin{matrix}{{{\cos \left( \theta_{e} \right)} = \frac{\gamma_{SV} - \gamma_{SL}}{\gamma_{LV}}},} & (1)\end{matrix}$

where θ_(e) is the equilibrium contact angle and, γ_(SV), γ_(SL), andγ_(LV) are, respectively, the solid-vapor, solid-liquid, and theliquid-vapor interfacial free energies per unit area. By assuming that aliquid completely fills grooves of a rough surface, the contact angle ofa droplet on a rough surface may be modeled as

cos(θ^(W))=r·cos(θ_(e)),  (2)

where r is a roughness factor defined as the ratio of an actual area ofthe rough surface to a projected area. Alternatively, by assuming thatthe liquid is completely suspended by the grooves, the contact angle ofa droplet on a rough surface may be modeled as

cos(θ_(r) ^(C))=f _(s)·(1+cos(θ_(e))−1,  (3)

where f_(s) is the ratio of the area of the rough surface contacting thedroplet to the projected area.

As noted previously, the presence of a surface wettability gradientinduces net mass transport of droplets. In particular, a droplet tendsto move toward the more wettable side if it experiences an imbalance incapillary forces across its edges, i.e., across the two opposite sidesof the liquid-solid contact lines. Wettability gradients can be used toremove droplets from the surface by creating a spatial variation in thephysical or chemical properties of the surface. As the droplet movesalong the surface, a resistance force is developed typically attributedto the presence of local defects. In a tilted-surface experiment inwhich a droplet is deposited on an initially horizontal plate and theplate tilted until the droplet is just about to start moving, the forcebalance provides a measure of contact-angle hysteresis. In particular,the contact-angle hysteresis may be specified by the resistance force

$\begin{matrix}{{{{\cos \left( \theta_{R} \right)} - {\cos \left( \theta_{A} \right)}} = \frac{m \cdot g \cdot {\sin (\alpha)}}{\varpi \cdot \gamma_{LV}}},} & (4)\end{matrix}$

where contact-angle hysteresis (θ_(R)−θ_(A)) is defined as thedifference between receding θ_(R) and advancing θ_(A) contact angles, gis the acceleration of gravity, α is the minimum angle of tilt at whicha droplet will spontaneously move, and m and ω are, respectively, themass and width of the droplet base. Thus, a successful, free-energydriven droplet transport on a roughened surface may be achieved bycreating an imbalance in capillary forces across the edges of thedroplet, and minimizing contact-angle hysteresis.

While drop-wise condensation may be the preferred regime of condensationbecause of its higher rate of heat transfer compared to the film-wisecondensation, initiating and maintaining drop-wise condensation is oftenchallenging. For example, initiating drop-wise condensation usuallyrequires non-wettable surfaces. In addition, maintaining drop-wisecondensation usually requires continuous removal of small droplets fromthe surface. In contrast, during film-wise condensation the condensedliquid tends to wet the surface, and a film of liquid is formed on thesurface by coalescence of the droplets. However, the presence of theliquid film typically significantly reduces heat transfer across thesurface. As a consequence, the rate of condensation typically decreasesas well.

In configurations where wettability is poor, the formation of a liquidfilm may be impeded, and the surface may be covered with a distributionof droplets with various sizes. This is drop-wise condensation. In thisregime, heat transfer between the surface and the humid air may only beeffected where droplets are present. Larger droplets, such as those withdiameters greater than 10 μm, have a large thermal resistance and, thus,behave locally like a liquid film, which can significantly hinder heattransfer. Therefore, drop-wise condensation often can quickly turn intofilm-wise condensation, reducing the effectiveness of heat transfer andthus the condensation process. The super-hydrophobic thin film and/orthe micro-patterned rings in the outer surface region on the surface ofthe sampler may be designed to prevent these challenges by initiatingand maintaining drop-wise condensation.

We now describe techniques for fabricating the micro-analyzer. Ingeneral, both chemical composition and physical roughness of the surfacecontribute to its wettability. In the micro-analyzer, the surface may bepatterned with micro-fabricated features, namely etched grooves or themicro-patterned rings, to increase its hydrophobicity. The wettabilitygradient may be obtained by gradually varying the roughness of themicro-patterned surface. Moreover, the surface may be subsequentlycoated with a low-energy interface material to make itsuper-hydrophobic. In an exemplary embodiment, silicon is used as thematerial of the sampler surface because of its high thermal conductivityand its adaptability with micro-fabrication techniques. Note that thesurface may be patterned using contact photolithography, and deepreactive-ion etching may be used to etch the characteristic groove/ridgestructure. The widths of the ridges and grooves may be varied tomodulate the surface wettability. For example, the widths of the ridgesand grooves may be changed radially to establish a wettability gradientin the radial direction of the sampler geometry. Next, the surface ofthe sampler may be spin-coated with a solution of polybutadienedissolved in toluene, which is then annealed in a vacuum to removeentrapped solvent. Furthermore, the coated surface may be plasma treatedin a vacuum chamber to create a plasma-fluorinated polybutadiene film.The fabrication of micro-analyzer 100 (FIGS. 1 and 2) is summarized inFIG. 3.

In an exemplary embodiment, a double-sided polished <100> silicon waferis baked at 110 C for 12.5 minutes to remove water moisture. Then, thewafer is spin-coated with an adhesive layer of hexamethyldisilazane(HMDS) primer followed by a photoresist, such as MEGAPOSIT™ SPR™ 220TM-7 photoresist (from Rohm and Hass Company of Philadelphia,Pennsylvania). Moreover, the wafer is soft-baked at 105 C for 360 sec.Next, the wafer is patterned with the pre-defined micro-features usingcontact photolithography. To degas nitrogen from the wafer, a three-hourdelay is allowed, followed by baking the wafer at 105 C for 300 sec.After a 45-minute delay, the wafer is softly agitated for 10-12 minutesin a developer, such as CD-26. The micro-features are then etched usinga deep reactive-ion etcher.

In order to minimize the hysteresis and ease the transport of dropletson the prepared surface, cleanness of the wafer may be maintained duringeach operation in the surface coating process. For example, the wafermay be cleaned with acetone, dried with nitrogen, and further cleanedusing oxygen plasma for 5 minutes. Polybutadiene having a molecularweight of 420 000, 36% cis 1,4 addition, 55% trans 1,4 addition, and 9%1,2 addition (from the Sigma-Aldrich Company, LLC of St. Louis, Mo.) isdissolved in Touline (99.5% purity) at concentration of 5% (w/w). Thewafer is then spin-coated with the prepared solution at 2000 rpm for 60seconds. Next, the thin film is annealed in a vacuum oven for 1 hour at900 C to remove entrapped solvent. The surface is plasma treated in avacuum chamber for ten minutes. Prior to each plasma treatment, thechamber may be scrubbed with isopropanol followed by further cleaningusing oxygen plasma at 200 Watts for 15 minutes. Furthermore, the waferis placed inside the chamber, and the chamber is pumped down to its basepressure of 25 mTorr. CF4 gas is then admitted into the chamber, and theelectrical discharge ignited. The plasma fluorination is carried out at150 mTorr, 60 Watts, and 3.0 sccm CF4 flow rate for ten minutes. Theplasma-fluorinated polybutadiene film is then cured in a vacuum oven forone hour at 900 C.

The resulting sampler surface may be micro-patterned with a series ofconcentric regions. Each region may include a periodic arrangement ofradially patterned ridges and grooves with the density of ridges andgrooves increasing (i.e., the widths of ridges and grooves decreasing)toward the center of the surface. Thus, the surface roughness may betuned by increasing the number of walls and grooves toward the center.While the desired contact-angle distribution may be increasingly lesshydrophobic toward the collection point, note that it may not be linearfor a droplet to move on the surface. In particular, the dropletmovement along the wettability gradient may be enhanced by theincreasing difference between subsequent contact angles as the size ofthe micro-patterns decreases. Thus, the decreasing sensitivity of thedroplet as the size of the micro-features decreases may be compensatedfor by an increasing difference in the contact angle. In addition, notethat the wettability gradient may be distributed not only in a discretemanner from one micro-patterned region to the other, but also along thelength of each region. This continuous distribution may be achieved byusing trapezoidal geometry, with the wide base oriented toward the inneredge for the micro-patterns on each region. The resulting non-lineardistribution of wettability gradient and the use of trapezoidal geometryfor the micro-patterns may provide a successful self-cleaning property,which may collect at least 7% more condensate than a plain hydrophobicsurface with the same dimensions. For example, the micro-analyzer maycollect 50 μL of condensate from exhaled human or animal breath in 5minutes.

In an exemplary embodiment, the collection point may be an un-patternedcircular area at the center. For example, the un-patterned circular areamay have a diameter of 8 mm, the grooves may be approximately 60 pmdeep, the sampler may have an outer diameter of 20 mm, and the substratemay have a thickness of 500 μm. Alternatively, the collection point maybe etched through the substrate so that it is open from the back side,which may allow a sample to be collected in a vial.

In experiments characterizing the surface of the sampler in amicro-analyzer having six sets of micro-patterned concentric rings orregions (with the geometry and the static contact angles summarized inTable 1), the contact angles of water droplets deposited on the surfaceof the sampler were measured with a goniometer. (However, in otherembodiments, there may be fewer or more sets of micro-patternedconcentric rings.) These contact angles were best described by Eqn. 3,i.e., the droplets deposited on the surface of the sampler are lifted bythe micro-patterns. As discussed previously, the wettability gradient isdistributed between the outer edge of the sampler surface (which is themost hydrophobic region) and the center collection point of the sampler(which is the least hydrophobic region). As a consequence, the measuredcontact angles on micropatterned regions decreased gradually from 157.0to 126.7° (i.e., from the most hydrophobic to the least hydrophobicregion) toward the center of the sampler surface. Moreover, the contactangle measured at the central collection point was 122°. (A greaterrange of differences can be achieved using similar techniques withnano-scale features.) The maximum measured contact-angle hysteresis(θ_(R)−θ_(A)) was approximately 1.8°. Thus, resistance for movement ofdroplets deposited on the surface was minimized.

TABLE 1 Number Groove of Groove Radius Wall Width Width and Wall RegionPosition (μm) (μm) (μm) Pairs θ_(e) (°) 1 Inward 4000 5.00 2.00 3590126.7 Outward 5000 4.50 4.25 2 Inward 5000 5.00 6.50 2732 131.2 Outward6040 4.50 9.30 3 Inward 5960 5.00 15.70 1821 139.4 Outward 7040 4.5019.65 4 Inward 6960 5.00 31.23 1214 147.4 Outward 8040 4.50 36.90 5Inward 7960 5.00 57.10 809 153.3 Outward 9040 4.50 65.36 6 Inward 89605.00 99.79 540 157.0 Outward 10000 4.50 111.94

(Note that, as illustrated in Table 1, the circular micro-patternedregions may be interconnected by overlapping one another, i.e. twoconsecutive regions may be chained as interlocking fingers.)

Moreover, motion of a series of purified water droplets (with sizesranging from 2-10 μL) deposited with a syringe on the surface of thesampler was captured using a video camera. As a droplet was deposited onthe surface it moved along the wettability gradient toward the center ofthe surface driven by the difference in capillary forces, which overcamethe induced resistance force.

In additional experiments, the behavior of the sampler surface undercondensation conditions was investigated. In condensation tests, thesurface of the sampler was actively cooled using a thermoelectric coolerto 16 C, while the surrounding environment was at atmospheric pressure,an ambient temperature of 22.2 C, and a relative humidity of 48%.Initially, droplets nucleated on the surface without significantinteractions among them. However, as the droplets grew in size, some ofthem coalesced and continued to grow in size to form larger droplets.After nucleation and initial growth, the droplets merged at theircontact lines with other droplets. As the droplets coalesced, newnucleation sites became available, allowing new nuclei to grow.Moreover, as the condensation progressed, the droplets continued growingin size. Once they reached a critical size (e.g., when the droplets‘felt’ the wettability gradient across their edges), they moved in adirectional manner along the wettability gradient.

Thus, two forms of droplet sweeping took place during the condensationprocess on the micro-analyzer. During ‘local sweeping,’ nucleateddroplets swept the surface as they grew in size and coalesced. Then,during ‘radial sweeping,’ larger droplets swept the surface andswallowed smaller droplets as they moved along the wettability gradienttoward the collection point. This radial sweeping cleared the smallerdroplets from the surface of the sampler, and therefore created newsites for droplet nucleation and growth. Because heat transfer is mostlysuppressed at locations where droplets have diameters larger than 10 μm,the droplet sweeping on the micro-analyzer was effective in removingdroplets from the surface during drop-wise condensation.

The preceding embodiments may include fewer components or additionalcomponents. For example, instead of the radial geometry described, themicro-analyzer may include a rectangular configuration of micro-pillars.Furthermore, instead of a trapezoidal cross-section, rectangular wallsmay be used in the radial geometry. Although these embodiments areillustrated as having a number of discrete items, these embodiments areintended to be functional descriptions of the various features that maybe present rather than structural schematics of the embodimentsdescribed herein. Consequently, in these embodiments two or morecomponents may be combined into a single component, and/or a position ofone or more components may be changed.

Note that the micro-analyzer may be fabricated using an additive orpositive process (i.e., a material-deposition process) and/or asubtractive or negative process (i.e., a material-removal process). Forexample, the process may include: sputtering, plating, isotropicetching, anisotropic etching, a photolithographic technique and/or adirect-write technique. Additionally, these processes may utilize a widevariety of materials, including: a semiconductor, metal, glass,sapphire, an organic material (such as polytetrafluoroethylene), aceramic material, a plastic and/or silicon dioxide.

We now describe the method. FIG. 4 presents a flow chart illustrating amethod 400 for analyzing droplets, which may be performed usingmicro-analyzer 100 (FIGS. 1 and 2). During this method, droplets arereceived in an outer surface region of a sampler surface disposed on asubstrate (operation 410). Note that the sampler surface may beincreasingly less hydrophobic along a radial direction toward the centerof the sampler surface. For example, the outer surface region may bemicro-patterned to create a wettability gradient which is distributedaxisymmetrically.

Then, the droplets are aggregated and moved radially toward the centerof the sampler surface (operation 412). This aggregation and motion mayoccur passively, i.e., a power source may not be used to transport thedroplets on the sampler surface.

Next, the droplets are received at an inner surface region of thesampler surface (operation 414). Furthermore, the droplets are analyzedusing an analysis mechanism (operation 416).

In some embodiments of method 400 there are additional or feweroperations. Moreover, the order of the operations may be changed, and/ortwo or more operations may be combined into a single operation.

In the preceding description, we refer to ‘some embodiments.’ Note that‘some embodiments’ describes a subset of all of the possibleembodiments, but does not always specify the same subset of embodiments.

The foregoing description is intended to enable any person skilled inthe art to make and use the disclosure, and is provided in the contextof a particular application and its requirements. Moreover, theforegoing descriptions of embodiments of the present disclosure havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present disclosure tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art, and the generalprinciples defined herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentdisclosure. Additionally, the discussion of the preceding embodiments isnot intended to limit the present disclosure. Thus, the presentdisclosure is not intended to be limited to the embodiments shown, butis to be accorded the widest scope consistent with the principles andfeatures disclosed herein.

What is claimed is:
 1. A micro-analyzer configured for on-chipmicro-analysis, comprising: a substrate; a patterned surface disposed onthe substrate, which further comprises: an outer surface regionconfigured to collect and transport liquid droplets radially toward acenter of the patterned surface; and an inner surface region within theouter surface region, wherein the inner surface region is configured toreceive the liquid droplets collected by the outer surface region; andan analysis mechanism in an area in the inner surface region configuredto analyze the collected droplets.
 2. The micro-analyzer of claim 1,wherein the patterned surface is configured so that the surface isincreasingly less hydrophobic along a radial direction toward the centerof the patterned surface.
 3. The micro-analyzer of claim 1, wherein theouter surface region is micro-patterned to create a wettability gradientwhich is distributed axisymmetrically.
 4. The micro-analyzer of claim 3,wherein the wettability gradient increases in an inward radialdirection.
 5. The micro-analyzer of claim 1, wherein the outer surfaceregion comprises a set of micro-patterned concentric rings.
 6. Themicro-analyzer of claim 5, wherein a given micro-patterned concentricring comprises a set of radially-oriented wall-groove pairs.
 7. Themicro-analyzer of claim 6, wherein the number of wall-groove pairs inthe given micro-patterned concentric ring increases from the outermostring toward the innermost ring.
 8. The micro-analyzer of claim 7,wherein the number of wall-groove pairs increases as a result of adecreasing width of the grooves from the outermost ring toward theinnermost ring.
 9. The micro-analyzer of claim 6, wherein the number ofmicro-patterned rings is configured to control a wettability gradient ofthe outer surface region.
 10. The micro-analyzer of claim 1, wherein awall within a given wall-groove pair has a trapezoidal cross-section.11. The micro-analyzer of claim 10, wherein a base of the wall has avarying width which increases in an inward radial direction.
 12. Themicro-analyzer of claim 1, further comprising a hydrophobic coatingdisposed on the outer surface region.
 13. The micro-analyzer of claim 1,wherein the sampler surface receives a gas-phase sample that condensesinto the liquid droplets.
 14. The micro-analyzer of claim 13, whereinthe outer surface region facilitates condensing of at least a componentin the received gas-phase sample into the liquid-phase droplets on thesampler surface.
 15. The micro-analyzer of claim 1, wherein thesubstrate includes a silicon substrate.
 16. A method for analyzingdroplets, the method comprising: receiving the droplets on an outersurface region of a sampler surface disposed on a substrate, wherein thesampler surface is increasingly less hydrophobic along a radialdirection toward the center of the sampler surface; aggregating andmoving the droplets toward the center of the sampler surface; receivingthe droplets at an inner surface region of the sampler surface; andanalyzing the droplets using an analysis mechanism in an area in theinner surface region.
 17. The method of claim 16, wherein the outersurface region is micro-patterned to create a wettability gradient whichis distributed axisymmetrically.
 18. The method of claim 17, wherein thewettability gradient increases in an inward radial direction.
 19. Themethod of claim 16, wherein the outer surface region comprises a set ofmicro-patterned concentric rings.
 20. The method of claim 16, whereinthe sampler surface receives a gas-phase sample that condenses into theliquid droplets.
 21. The method of claim 20, wherein the outer surfaceregion facilitates condensing of at least a component in the receivedgas-phase sample into the liquid-phase droplets on the sampler surface.